Introducing feed into externally circulating electrolyte in electrochemical process

ABSTRACT

In an electrochemical operation, e.g., fluorination of an organic compound, the electrolyte is removed from the electrochemical cell, optionally circulated through a cooler, and returned to the cell. The feed is introduced into the electrolyte externally of the cell. The electrolyte, containing the feed, is introduced into the bottom portion of the cell which can be a multiple electrodes-containing cell in view of the feed introduction. Optimum design, disregarding substantially entirely heat buildup, is made possible. Fluorination of an organic compound is described.

This invention relates to electrochemical cell construction andoperation. In one of its aspects, the invention relates to the manner ofintroducing feed into the electrolyte during the operation of anelectrochemical cell, e.g., a cell in which electrofluorination is beingeffected.

In one of its concepts, the invention provides an electrochemical cellstructure having a means for circulating electrolyte from the cellduring the operation of the cell and means by which said electrolyte canbe recycled to the cell means for introducing feed to said electrolytewhile it is outside said cell, and means for introducing saidelectrolyte, together with entrained feed material or by-products whichmay be contained in the electrolyte to said cell. In another of itsconcepts, the invention provides a multiple electrode electrochemicalcell employing external circulation of electrolyte as herein described.In a further concept of the invention, the structure provides forintroduction of a circulating electrolyte containing feed added to itinto a lower portion or bottom of the cell structure.

BACKGROUND OF THE INVENTION

Electrochemical fluorination is a process whereby the passage of anelectrolytic current is made to incorporate fluorine into a substrate byaddition or substitution. This process can be used to produceperfluorocarbons, a class of compounds or substances known for theiroutstanding chemical, electrical, and thermal stability. It can also beused to produce materials which are reactive and useful as chemicalintermediates; these products include hexafluoroacetone, perfluoroacylfluorides, and perfluoroesters. Other products include sulfurhexafluoride and carbonyl fluoride.

The passage of this electrolytic current (electrolysis) is effectedthrough an electrolyte which usually is liquid hydrogen fluoride orhydrogen fluoride complexed with a current-conducting additive such asan alkali metal fluoride. This current is passed between a cathode and aporous anode contained in an electrolysis cell; hydrogen gas is evolvedat the cathode; and fluorine is generated at the anode. The fluorinereacts with the material to be fluorinated while it is within the poresof the anode.

It is known that production rate is contained in large part by thedensity of the current within the electrolyte solution, which for anormal one-cell unit, such as shown in U.S. Pat. No. 3,882,001, is about2.5 to 3.0 K amp/m². The disclosure of the patent is incorporated bythis reference. Increasing the current density with a correspondingincrease in feed rate increases production rates, but it also increasestemperature within the cell frequently beyond the capabilities of theheat-removing apparatus of the cell. Mass transfer is an important partof heat transfer, and, in the usual electrochemical fluorinationapparatus, this mass transfer is carried out by thermal siphon and byhydrogen lift.

Although fairly efficient, the process is hazardous because of the toxicand corrosive nature of hydrogen fluoride. This makes maintenancedifficult during cell repair. With certain cell configurations, feedlines become plugged with salts when the feed is passed from the bottomof the cell through the electrolyte into the anode area. When the feedis passed through the top of the cell down into the anode area, pluggingat least at the anode is avoided.

An object of this invention is to provide an electrochemical object.Another operation of this invention is to provide an electrochemicalcell structure. Still another object of this invention is to provide afeed introduction means permitting improved design and multipleelectrodes containing cell structure. A further object of this inventionis to provide such a multiple electrode electrochemical cell structure.A further object of this invention is to provide an optimum designedelectrochemical cell structure which can be designed disregardingproblems due to evolution of heat. A further object of the invention isto provide a structure for an electrochemical cell such that plugging ofthe cell is entirely avoided.

Other aspects, concepts, objects, and the several advantages of theinvention are apparent from a study of this disclosure, the drawing, andthe appended claims.

According to the present invention, there is provided a process forconducting an electrochemical reaction in an electrochemical reactionzone or cell which comprises circulating at least a portion of theelectrolyte in said zone from said zone, adding feed to cooling saidelectrolyte, and then returning at least a portion of the electrolytecontaining said feed to said reaction zone.

Still according to the invention, there is provided an electrochemicalcell having multiple electrodes therein and means associated with saidcell for cooling the electrolyte externally of said cell.

Further according to the invention, there is provided a process forconducting an electrochemical reaction in an electrochemical reactionzone or cell having multiple electrodes therein which comprisescirculating at least a portion of the electrolyte in said zone from saidzone to a cooling zone, in said cooling zone cooling said electrolyte,and then returning at least a portion of the thus-cooled electrolyte tosaid reaction zone.

Still further according to the invention feed added to the circulatingelectrolyte is returned to the area at the bottom of the anodes in thecell for passage upwardly through the anodes.

In an embodiment according to the invention there is provided, see FIG.1, a feed disengaging area at the bottom of the anode to ensure thatfeed will disengage from the electrolyte and pass upwardly through theanode area while the electrolyte, upon disengagement of the feed, ispassed through the space or area between an anode and a cathode.

In a specific embodiment of the invention, there is provided anelectrochemical fluorination operation of a cell structure according toan embodiment of the invention.

Advantages of the process of the invention are as follow:

1. The feed is introduced through the bottom of the multi-cell unit.Thus the top of the assembly is less cluttered as are otherconfigurations, making maintenance operations simpler and safer. Thiscan be and in the now preferred embodiment is done by introducing thefeed with the cooled, recycled electrolyte as described below. Theseadvantages are made possible by the introduction of the feed into theelectrolyte.

2. The electrolyte is circulated and cooled externally to the cell usinga pump and external heat exchanger. Such pumping permits faster flow ofthe electrolyte than can be obtained with the usual thermal siphon andhydrogen gas lift. This higher flow will permit operation at a highercurrent density at the same voltage, or at the same current density witha lower voltage. Thus, one can obtain higher productivity or lowerelectrical energy consumption. The invention permits a simpler, morecompact cell design since no provision need be made for internal heatexchange surface or down-comer tubes. It also permits simple filteringof the electrolyte to remove the sludge that accumulates in celloperation.

3. It is simpler to accurately split a large volume stream ofelectrolyte containing dispersed feed into several streams, for feedingindividual streams for such feeding. The small orifices ordinarilyneeded to split just the feed stream when not added to the electrolyteas is done in the present invention, are prone to plug and corrode asthe recycle feed frequently contains considerable HF and, on occasion,some heavy material.

The single cell data used herein to illustrate the invention are basedon the process described by R. B. MacMullin et al., J. Electrochem.Soc., Vol. 118, No. 10, 1582 (1971), and U.S. Pat. No. 3,711,396. Porouselectrodes of a particular kind for use in electrochemical processes aredescribed and claimed in U.S. Pat. No. 3,558,450. These disclosures areincorporated herein by reference.

FEEDSTOCK MATERIALS

Feedstock materials to which this invention can be applied include anytype of materials suitable for electrochemical fluorination, such ashydrocarbons (paraffinic or aromatic, saturated or unsaturated),oxygenated compounds (ketones, esters, acids, alcohols, ethers, carbonmonoxide, carbon dioxide), halogenated compounds (chloroform,chloroethane, dichloroethane), or any other material known to be capableof electrochemical fluorination.

ELECTROLYTE

The electrochemical fluorination process is carried out in a medium ofhydrogen fluoride electrolyte. Although the hydrogen fluorideelectrolyte can contain small amounts of water, such as up to about fiveweight percent, it usually is preferred that the electrolyte beessentially anhydrous. The hydrogen fluoride electrolyte is consumed inthe reaction and must be either continuously or intermittently replacedto maintain constant composition and operating bath levels.

Pure anhydrous liquid hydrogen fluoride is nonconductive. Theessentially anhydrous liquid hydrogen fluoride described herein has alow conductivity which, generally speaking, is low than desired forpractical operation. To provide adequate conductivity in theelectrolyte, and to reduce the hydrogen fluoride vapor pressure at celloperating conditions, an inorganic additive can be and usually isincorporated in the electrolyte. Examples of suitable additives areinorganic compounds which are soluble in liquid hydrogen fluoride andprovide effective electrolytic conductivity. The now usually preferredadditives are the alkali metal (sodium, potassium, lithium, rubidium,and cesium) fluorides and ammonium fluoride. Other additives which canbe employed are sulfuric and phosphoric acid. Potassium fluoride, cesiumfluoride, and rubidium fluoride are the preferred additives. Potassiumfluoride is a particularly preferred additive. The additives can beutilized in any suitable molar ratio of additive to hydrogen fluoridewithin the range of from 1:4.5 to 1:1, preferably 1:4 to 1:2. Thepresently preferred electrolytes include those which correspondapproximately to the formulas KF.2HF, KF.3HF, or KF.4HF. Suchelectrolytes can be conveniently prepared by adding the requiredquantity of hydrogen fluoride to KF.HF (potassium bifluoride). Ingeneral, said additives are not consumed in the process and can be usedindefinitely. A small amount of lithium fluoride is also sometimesadded. Said additives are frequently referred to as conductivityadditives for convenience. It is in the use of such electrolytes as heredescribed that the invention is especially applicable.

The cell body, cathode, anode, and the configuration of theelectrochemical cell unit are known in the art. Suffice here to notethat the gap or space designed into such units is such that cooling ofthe electrolyte internally of the unit is not practical.

FIG. 1 is a diagrammatic view of an individual electrode unit.

FIG. 2 is a flow diagram showing an operation according to theinvention.

FIG. 3 is a multiple electrode cell set forth for use according to theinvention.

The electrode configuration as shown in FIG. 1 is not intended to limitthe size of the electrodes but merely to serve as an illustration of onetype arrangement. The broad range for the gap distance between the anodesurface and the cathode surface is 0.254 × 10⁻³ m (0.01 inch) to 0.254m(10 inches) with a usually preferred range of 0.51 × 10⁻³ m (0.02 inch)to 0.0254m (1.0 inch).

EXTERNAL COOLING AND CIRCULATING UNIT

FIG. 2 shows the circulation route an external cooling of an electrolytesolution described herein and the addition of the feed to theelectrolyte while it is outside the cell or reaction zone. Any type pumpcapable of circulating electrolyte can be used for this invention.

The invention permits circulating, as rapidly as desired and as possiblea large amount or volume of electrolyte to which the feed is added,without regard to cell size or structure. Likewise, any type heatexchanger capable of cooling said electrolyte can be used for thisinvention. The pump, heat exchanger, and all lines to and from theseunits should be constructed of materials that will resist any corrosivenature of the electrolyte. When the electrolyte is composed of potassiumfluoride or hydrogen fluoride, the temperature of the coolant in theheat exchanger should be maintained above about 60° C. so as to preventcrystallization which as noted earlier occurs at about 58° C. Thecoolant can be any convenient liquid suitable as a heat exchange medium,such as water, methanol, glycol, and the like.

PROCESS OPERATING CONDITIONS

Temperature

When fluorinating an organic, it is now preferred to operate attemperatures such that the vapor pressure of the electrolyte will beless than about 50 mm Hg. As will be understood by those skilled in theart, the vapor pressure of the electrolyte at a given temperature willbe dependent upon its composition. It is well known that additives suchas potassium fluoride cause the vapor pressure of liquid hydrogenfluoride to be decreased an unusually great amount. A presentlypreferred range of temperature is from about 60° C. to about 110° C.

Pressure

Pressures substantially above or below atmospheric can be employed ifdesired, depending upon the vapor pressure of the electrolyte asdiscussed above. Broadly a range of 0.5 to 3.0 atmospheres can beemployed with a preferred range of about 0.9 to 1.5 atmospheres. In allinstances, the cell pressure should be sufficient to maintain a liquidphase of electrolyte and also such as to provide for easy removal of theproducts.

Current Density

For purposes of efficiency and economy, the rate of direct current flowthrough each electrode is maintained at a rate which will give thehighest practical current densities for the electrodes employed.Generally speaking, the current density should be high enough so thatanodes of moderate size can be used, yet low enough so that said anodesare not corroded or disintegrated under the given current flow. Currentdensities within the range of from 0.3 to 10.0 or more, preferably 1.0to 4.0 kiloamps per square meter of each anode geometric surface areacan be used. Current densities less than about 0.3 KA/m² of anodegeometric surface are not practical because the rate of fluorination istoo slow. The voltage which is employed will vary depending upon theparticular cell configuration employed and the current density employed.Voltages in the range of from 4 to 12 volts per electrode are typical.The maximum voltages during depolarization will reach about 80 volts.Thus, as a guide in practicing this invention, voltages in the range of4 to 12 volts per electrode can be used for normal operations. Thepresent invention permits the designer to design the best or optimumcell without regard to heat evolved to the extent that the externalcooling provided will cope with such heat.

Feed Rate

Feed rates which can be employed in the practice of this inventiondepend in part upon several things among which are the type of carbonused in the porous anode, the number of electrodes employed in thecomplete cell, and the nature of the material being fluorinated. Becauseof the wide nature of materials that can be employed in this invention,feed rates are thought of in terms of how much hydrogen is replaced withfluorine in a molecule of feed. Therefore, a broad feed rate range toeach individual anode can be 5 to 125 percent per pass replacement ofhydrogen equivalent with a preferred range of 20-90 percent per passreplacement of hydrogen equivalent. Using ethane as in the example, thiswould be interpreted as a broad range of 33 to 835 liters per hour and anarrow range of 55 to 334 liters per hour.

In the preferred method of practicing this invention, the feed rate willbe such that the feedstock is passed into the pores of the anode, andinto contact with the fluorinating species therein, at a flow rate suchthat the inlet pressure of said feedstock into said pores is essentiallyless than the sum of (a) the hydrostatic pressure of the electrolyte atthe level of entry of the feedstock into said pores and (b) the exitpressure of any unreacted feedstock and fluorinated products from saidpores into the electrolyte. Said exit pressure is defined as thepressure required to form a bubble on the outer surface of the anode andbreak said bubble away from said surface.

Again, the virtual elimination of the heat problem will permit thedesigner to better concentrate with greater flexibility upon the optimumfeed rate, etc.

EXAMPLE I -- PRIOR ART

This is a calculated example of a single electrode containingelectrofluorination cell based on experience with fluorination cellshaving similar structure.

A porous carbon anode is fabricated from National Carbon 45, having anaverage pore size of about 55 microns, a permeability of about 20darcys, and a total porosity of about 50% and is a cylinder measuring0.673m (26.5 inches) × 0.2m (7.8 inches). A cavity, 0.152m (6.0 inchdiameter) × 0.019m (0.75 inch deep) is cut into the bottom of the anode.Positioned around the anode is a circular iron cathode measuring 0.637m(25.1 inches) × 0.22m (8.66 inches) ID with 0.006m (0.25 inch) thickwalls. The distance between the outer anode surface and inner cathodesurface (gap distance) is 0.01m (0.39 inches).

The above-described electrode is placed in an electrochemical conversioncell which contains KF.2HF as the electrolyte maintained at about100°-105° C. An ethane feed tube of 0.0047m (0.187 inch) copper tubingencased in a 0.006m (0.25 inch) Teflon tubing is provided. This tubingextends to the bottom of the electrode and feeds ethane directly intothe electrode cavity. The anode is submerged 0.658m (25.9 inches) intothe electrolyte and, during operation, the fluorinated products andunconverted feed material leave the electrode through the portion of theporous carbon above the surface of the electrolyte. Hydrogen is evolvedat the cathode.

The following operating conditions would normally be expected to prevailin a single operation as described in this example.

    ______________________________________                                        Electrolyte circulation                                                                            5.0 × 10.sup.3 liters/hr                           (through Anode-Cathode 0.01m Gap)                                             Ethane feed rate at 25%                                                                            167 liters/hr                                            per pass replacement of hydrogen                                                                   (7.47 moles/hr)                                          Current              1.2 kiloamps                                             Voltage              ˜9 volts                                           Heat evolved         10.8 kilowatts                                           Temperature          105° C.                                           ______________________________________                                    

Product distribution is as follows:

    ______________________________________                                                            Mole Percent                                              ______________________________________                                        CH.sub.3 -- CH.sub.3  23.3                                                    CFH.sub.2 -- CH.sub.3 7.5                                                     CFH.sub.2 -- CFH.sub.2                                                                              5.8                                                     CF.sub.2 H -- CH.sub.3                                                                              3.9                                                     CF.sub.2 H -- CFH.sub.2                                                                             12.9                                                    CF.sub.3 -- CH.sub.3  2.4                                                     CF.sub.2 H -- CF.sub.2 H                                                                            10.5                                                    CF.sub.3 -- CFH.sub.2 7.0                                                     CF.sub.3 CF.sub.2 H   9.4                                                     CF.sub.3 -- CF.sub.3  13.9                                                    CF.sub.4              0.6                                                     C.sub.3 + flourides   2.8                                                     Ethane Conversion, %  77.1                                                    Current efficiency, % 100.0                                                   ______________________________________                                    

CALCULATED ILLUSTRATION -- EXAMPLE II

This is a calculated example of a 36 electrode containingelectrofluorination cell. The electrodes are the same size as describedin example I and are arranged in the cell unit as shown in FIG. 3,although any convenient or practical arrangement can be used with anynumber of electrodes as desired. In this example, electrodes aresubmerged the same depth as for the single electrode cell. The pipesfrom the electrochemical cell to the external heat exchanger and back tothe cell must accommodate circulating electrolyte and any entrainedorganic or in this example ethane on the return to the cell. Theelectrochemical fluorination process at each anode-cathode surface isthe same for the 36-electrode unit as for the single electrode unit,that is, reactant is converted to a fluorination product in the anodearea. The product, unreacted reagent, and hydrogen by-product is removedthrough the top of the electrochemical fluorination cell. As noted, FIG.2 shows, according to the invention, the general operation, in which theelectrolyte is passed from the cell unit through a circulating pump,into a heat exchanger, back to the fluorination cell, and finallythrough the gap area between the anode and cathode. Ethane is fed intothe circulating electrolyte liquid between the heat exchanger and thefluorination cell. The ethane is insoluble in the electrolyte and thusreadily separates in the cap area at the bottom of the anodes forpassage through the anodes, the electrolyte not passing through theanodes but rather going through the area between each anode and cathode.

The above-described multi-electrode cell, fluorination, and circulationprocess is calculated to be carried out under the following operatingconditions.

    ______________________________________                                        Electrolyte circulation                                                       (through Anode-Cathode 0.01m gap)                                                                  5. × 10.sup.3 liters/hr                            Electrolyte circulation                                                                            1.8 × 10.sup.5 liters/hr                           (through heat exchanger)                                                      Ethane feed rate at 25%                                                                            6. × 10.sup.3 liters/hr                            per pass replacement of hydrogen                                                                   (268 moles/hr)                                           Current              43 kiloamps                                              Voltage              ˜9 volts                                           Heat evolved         390 kilowatts                                            Temperature          105° C.                                           ______________________________________                                    

Product distribution is the same as shown in example I except the rateat which product is produced is multiplied by the number of electrodesused in the cell; which is made possible by the invention.

During the process hydrogen fluoride is consumed. As HF is consumed,KF.HF forms as a by-product that tends to precipitate during operationsbut is partially solubilized by the KF.xHF (x = 2, 3, or 4) plus itssweeping action through the feed lines prevents any plugging to occur inthe feed lines. Hydrogen fluoride is periodically added to theelectrolyte solution to convert the by-product KF.HF back to theelectrolyte KF.xHF.

Care should be taken during the operation to maintain the temperatureabove about 60° C. The electrolyte, KF.2HF has a freezing point about58° C. that could cause plugging if allowed to cool below about 60° C.For this reason the electrolyte circulating lines are insulated toassist in maintaining the electrolyte temperature above about 60° C.

Usually, when fluorinating, as with an electrolyte as herein described,the temperature thereof will vary in the approximate range of from about110° C. in the cell to about 60° C. after it is cooled and prior to itsreturn to the cell. Obviously the temperature in the cell and that towhich the electrolyte is cooled prior to its recycle to the cell willdepend upon the nature of the electrochemical reaction being conductedand upon the electrolyte used.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure, the drawing and the appended claims to theinvention the essence of which is that by providing externalintroduction of the feed into the electrolyte and circulation thereof tosaid cell admixed with said electrolyte plugging is avoided and theelectrolyte is fed back to the cell, optionally after cooling, to abottom portion thereof, with a facilitated, much improved distributionof feed to each anode in a multiple cell or reaction zone.

I claim:
 1. A process for conducting an electrochemical fluorinationreaction upon an organic compound in an electrochemical fluorinationreaction zone or cell containing a plurality of anodes which comprisescirculating at least a portion of the electrolyte in said zone from saidzone, adding to the electrolyte which has been removed from said zone,the organic compound feed for said electrochemical fluorinationreaction, subdividing the mixture of feed and electrolyte thus obtained,and then returning a portion of said electrolyte now containing saidfeed to each of the anodes in said reaction zone.
 2. A process accordingto claim 1 wherein the electrolyte and feed mixture returned to saidreaction zone is introduced into the bottom portion of said reactionzone.
 3. A process according to claim 1 wherein the feed is an organiccompound immiscible with said electrolyte and the disengaged feed ispassed through the anode.
 4. A process according to claim 1 wherein thetemperature of the electrolyte in the electrochemical fluorinationreaction zone is of the order of not more than about 110° C. and coolingis effected to reduce that temperature to about 60° C. before recyclingthe cooled electrolyte and its contained feed to the reaction zonewherein an electrofluorination process is being conducted and theelectrolyte is prepared by adding hydrogen fluoride to KF.HF.
 5. Aprocess according to claim 1 wherein the electrolyte and the feedadmixed therewith is subdivided and is fed into a reaction zone to justbelow each of a plurality of anodes in said reaction zone, wherebydeposits in said zone at the entry of said zone of the electrolyte ofsolid decomposition product stemming from the electrolyte are avoided.